Gnomes are cool. They never tried to exterminate any other race. They dug a bit too deep but they could not have known. They helped their friends, allies whilst their own homeland was being overrun by vermin and they were betrayed from within.
Dwarves... well, there have been clashes with other native populations when they went out in the world but it was nothing like the Draenei genocide.
Really, these two cousin-races pretty much disprove you.